For many years the very high contrast photographic images needed in the graphic arts and printing industries were obtained by developing a `lith` emulsion (usually high in silver chloride content) in a hydroquinone, low sulphite, `lith` developer by the process known as infectious development. However, such low sulphite developers are inherently unstable and are particularly inappropriate for machine processing.
More recently, emulsions containing hydrazine nucleating agents have been used and processed in a high pH (about pH 11.5) developer with conventional amounts of sulphite, hydroquinone and possibly metol or a pyrazolidone. While such a process is better than the low sulphite lith process, the developer still has less sulphite than is optimal and a high pH requirement for it to function correctly. Such a solution is not as stable as is desirable. Additionally, high pH solutions are environmentally undesirable because of the care needed in handling and disposing of the effluent.
EP-A-0 531 014 claims high contrast materials sensitive to more than one spectral region comprising one layer of emulsion sensitive to one region and another emulsion layer sensitive to another region. Each spectral sensitivity requires its own emulsion layer. In a comparative test (page 6 lines 9-19), a blend of differently dye sensitised emulsions was used as the sole emulsion layer. This material showed loss of speed, especially if one of the spectral regions is in the infra red. The explanation for this appears to be that the sensitising dyes are being desorbed from their silver halide grain hosts.
EP-A-0 208 514 claims high contrast materials containing a hydrazide wherein there are two distinct populations of grains differing in grain volume. In the comparative examples (outside the scope of the invention claimed) emulsion blending is described. Some of the blends are dye sensitised. However, the blending takes place before the addition of any sensitising dye so that the blended emulsions described have grains which are either all dye sensitised or all not dye sensitised. Blends of sensitised and unsensitised grains are not mentioned.
A further improvement in the area of high contrast materials is the introduction of a lower pH process (below pH 11) using hydrazides active at this low pH together with the use of a contrast booster compound, for example, one of the boosters as described in U.S. Pat. No. 5,316,889 or an amine booster as described in U.S. Pat. Nos. 4,269,929, 4,668,605 and 4,740,452. The hydrazides proposed for use in such materials are described, for example, in U.S. Pat. Nos. 4,278,748, 4,031,127, 4,030,925 and 4,323,643 and in European Patent 0,333,435.
In most photographic materials the type and size of the silver halide grain determines the speed of the material while also affecting the covering power of the silver image formed therefrom. In general smaller sized grains provide higher density and covering power than larger ones. In some materials therefore, there has to be a balance struck between speed and covering power. In high contrast materials another balance between vigorous development and pepper fog (which occurs if development is too vigorous) needs to be achieved.